I've never really had this problem, but all my pants are shrinking in length. It's really frustrating, because all of my pants are shrinking, not just one pair, so my ankles always tend to stick out the bottom.
You don't give enough information to suggest one answer. If you are a teenager, that is normal. If you have all cotton jeans and you just changed your laundry to hot water, that is normal. It might be something else that I can't guess.